Saturday, February 17, 2018

New wind and wave criteria changes threshold for potential closures of the westbound I-90 floating bridge

FINAL UPDATE 6:39 p.m. Sunday: All lanes of the I-90 floating bridge have reopened to traffic. Crews inspected the bridge after conditions died down.UPDATE 2:18 p.m. Sunday, Feb. 18: Crews have temporarily closed the westbound I-90 floating bridge to traffic due to high winds & waves. All westbound traffic must exit at Island Crest Way on Mercer Island. There’s currently no estimated time for reopening. Eastbound I-90 remains open.

By Annie Johnson and Harmony Weinberg

Changes to how we react to wind and wave heights on the I-90 Homer M. Hadley Floating Bridge could mean a temporary closure on Sunday. This bridge carries westbound I-90 traffic, the future Sound Transit light rail tracks and the I-90 Trail.

If we hit a new threshold of sustained winds from the north of at least 26 mph (that north detail is very important and does not happen often) for two minutes and observe waves of at least 2 feet, we will not allow traffic onto the westbound I-90 bridge between Mercer Island and Seattle. Emergency vehicles will have access to the bridge at all times.

Why the change?
Wind and wave analyses performed during Sound Transit's East Link design process showed the pontoons on the westbound I-90 bridge are susceptible to damage during strong north wind events.

It's important to note that there is no immediate safety risk to the public or the bridge. However, any damage caused by strong north winds can shorten the life of the bridge.

Our Bridge Preservation Office used the analysis from Sound Transit to determine the new criteria to preserve the life of the westbound I-90 floating bridge. The new criteria requires us to close the westbound I-90 bridge to traffic when we have the following items occur simultaneously: sustained winds of at least 26 mph from the north for two minutes AND the winds create waves that are at least 2 feet tall.

How common is the new wind and wave combination?
Looking back over the past 10 years, there were eight days where wind from the north exceeded 26 mph on Lake Washington. Of those eight days, there was only one day where the sustained winds were sufficient to create a 2 foot wave.

Since we must have both sustained winds AND waves of at least 2 feet to close the bridge, it's likely we would have closed the bridge once in the past 10 years had the criteria been in place. We expect a potential closure due to the new wind and criteria to happen once every five to 10 years. A rare event. However, we are preparing and planning for it.

What's the fix?
One of the more permanent ways to prevent any damage from strong north winds is to make the bridge stronger through a construction technique known as post-tensioning, which is currently happening as part of Sound Transit's East Link project.

Post-tensioning is a common technique to compress and strengthen concrete structures, especially bridges. Contractor crews thread large steel cables through the inside of the pontoons and then tighten the cable to create one large rigid structure. This work will extend the life of the bridge by strengthening the existing pontoons, which lessens the chances of damage. The post-tensioning work will allow the bridge to withstand stronger winds and higher waves and is expected to be complete in fall of 2018. When that work is complete the wind criteria will
be reevaluated.

How much notice will travelers receive in the event of a westbound I-90 closure?
We would like to give as much notice as possible to drivers about potential closures, however, we will not close the road until all the criteria is met. Once that happens, we can close the roadway in about 15 minutes. We will use social media, our mobile app and overhead message
signs to alert travelers.

As is the case with any emergency closure, whether it's due to multiple spin outs on Snoqualmie Pass or a major collision, we all need to be prepared for delays whenever we hit the road. We will continue to do our best to keep you informed of any and all closures that could affect you. In the Seattle and Mercer Island areas, you can get real-time traffic information on your phone with the WSDOT traffic app, track the WSDOT Traffic Twitter feed and check out our Facebookpage.

The backstory
I'm sure you're wondering how we got here and why the bridge hasn't closed in the past when we've had high winds.

The westbound I-90 floating bridge was designed in the late 1970s and built in the 1980s and, like our hairstyles and clothes, many things have changed in the world of floating bridge engineering since then. If we were to build a brand new I-90 bridge today, it would likely look significantly different that the I-90 floating bridges you drive across today.

It's not that today's bridge is unsafe or needs to be replaced but we've learned a lot over the past 40 years. We now have much higher 100-year- storm criteria and know more about local storm and wind patterns.